Field of the Invention
The present invention relates to a liquid crystal display panel and a method for manufacturing liquid crystal display panel.
Description of the Background Art
As a displaying method for liquid crystal display panels, the twisted nematic (TN) mode is widely used. The TN mode is the vertical electric field mode in which liquid crystal molecules are driven by generating substantially perpendicular electric fields in the panels. Meanwhile, in recent years, the transverse electric field mode in which liquid crystal molecules are driven in the horizontal direction by generating substantially horizontal electric fields in the panels has also been used. The transverse electric field mode offers advantages such as wide viewing angle, higher resolution, and high luminance, thus becoming mainstream for small or middle-sized panels typified by smartphones and tablets. The typical example of the transverse electric field mode is the fringe field switching (FFS) mode.
When a voltage is applied between the pixel electrode and the counter electrode (common electrode) so that the transverse electric fields are generated, parasitic capacitance is generated between the signal line and one of the electrodes (lower electrode) that is located closer to the substrate. The large parasitic capacitance is likely to cause deterioration in display quality. To reduce the parasitic capacitance, the insulating film between the lower electrode and the signal line desirably has a large thickness and a small relative dielectric constant. In this respect, the insulating film is preferably an organic insulating film. The organic insulating film covers steps such as thin film transistors (TFTs), thus also being advantageous in terms of offering leveling properties. In a case where a material having photosensitivity is used as the organic insulating film, an opening (contact hole) can be directly formed therein by photolithography.
In Japanese Patent Application Laid-Open No. 2007-226175, the liquid crystal device to which the FFS is applied is disclosed. In one aspect described therein, the liquid crystal device includes relay electrodes extending into the contact holes in the insulating film made of SiO2, namely, the inorganic insulating film. The TFTs and the pixel electrodes are electrically connected by the relay electrodes. In this technique, the structure and the manufacturing method are complicated because the relay electrodes are included. Thus, the direct connection between the pixel electrodes and the drain electrodes without using the relay electrodes is desired.
United States Patent Application Publication No. 2013/0063673 discloses that the organic insulating film having photosensitivity is included as the insulating film disposed on the TFTs. The organic insulating film has the openings through which the TFTs are exposed. The pixel electrodes are electrically connected to the TFTs through the openings. Specifically, the pixel electrodes are in direct contact with the side surfaces of the ohmic contact layer and the active layer and with the drain electrodes.
In United States Patent Application Publication No. 2013/0063673, the portion of the pixel electrode that is effective in terms of generating electric fields, that is, the portion opposed to the common electrode extends above the opening of the organic insulating film, on the gate insulating film, on the side surface of the active layer, and on the side wall of the ohmic contact layer, thereby being connected to the drain electrode of the TFT. The pixel electrode follows such a complicated path, so that the pixel electrode may be interrupted at some midpoint, particularly on boundaries between the layers, due to variations in the manufacturing processes. Therefore, the manufacturing yields can be reduced.